Chances are that a computer acting as a home server is going to have quite a few hard drives. Storage, afterall, is what the home server is all about.
Hard drives are not typically power hogs, but when you have three or four they can consume a fair amount of power, perhaps up to 20 or 25 watts.
This power consumption can be reduced by using low-power hard drive. These hard drives, such as the Western Digital Green series, usually spin at lower speeds. They provide less performance, but consume less power. Another way to reduce power consumption is to purchase one or two very large hard drives instead of multiple smaller ones. For example, two terabytes of data could be handled by two one terabyte drives instead of four 500 gigabyte drives.
For more extreme power savings, solid state hard drives can be used. Solid state hard drive use very little power relative to hard disks, particularly at load. However, solid state drives are still expensive, so this isn't going to be in the budget of most people building a green home server.